Maleimide compounds having improved in storage stability

ABSTRACT

The present invention provides a stabilized maleimide compound which is hard to discolor during storage. The maleimide compound has a content of primary amines of not more than 500 ppm or further has a content of maleic anhydride in the range of 5 to 2,000 ppm or a content of 2-amino-N-substituted succinimide compounds of not more than 300 ppm. Furthermore, it is preferable that the content of chlorine compounds reduced as a chlorine atom is not more than 10 ppm and further the content of volatile components having a boiling point of not more than 200° C. at normal pressure is not more than 2,000 ppm.

TECHNICAL FIELD

The present invention relates to a maleimide compound with improvedstorage stability which is hard to discolor. More particularly, itrelates to a maleimide compound useful as a raw material for producing aheat resistant polymer such as AS resin, ABS resin and the like.

BACKGROUND ART

A maleimide compound is solid or liquid at room temperature so it isstored in various forms. For example, a solid maleimide is handled inthe form of tablets, flakes, powders, etc. at room temperature and theforms thereof during storage such as flake container bags, drumcontainers, gallon containers, paper bags, tank containers are varied.Maleimide compound in heated molten state is stored in storage tanks ina liquid state.

There has been a problem that the color of the maleimide compound ischanged gradually to a blown color, and sometimes into a slightly bluishblack brown color, when it is stored in such a solid or liquid state.The rate of discoloration of the maleimide compound varies depending onstate, form and storage conditions of the stored maleimide compound, andthe discoloration of the maleimide compound during its storage isinevitable although it can occur quickly or slowly.

Discoloration of the maleimide compound is an important problem whenused. That is, since the maleimide compound itself is a kind of rawmonomer used for improving heat resistance of various thermoplasticresins such as styrene type resins, e.g., AS resin and ABS resin, methylmethacrylate resins, and vinyl chloride resins, etc., the discolorationof the maleimide compound makes the various thermoplastic resinsdiscolored, and as a natural result, the commercial value of thethermoplastic resins decreases considerably. Thus it is necessary tosolve the discoloration problem just mentioned above.

Accordingly, an object of the present invention is to provide astabilized maleimide compound which is hard to discolor during itsstorage.

DISCLOSURE OF INVENTION

The above object can be attained by a maleimide compound with improvedstorage stability wherein the content of primary amines is not more than500 ppm.

The above object can also be attained by a maleimide compound withimproved storage stability wherein the content of primary amines is notmore than 500 ppm and the content of maleic anhydride is in the range of5 to 2,000 ppm.

The above object can further be attained by a maleimide compound withimproved storage stability wherein the content of primary amines is notmore than 500 ppm and the content of 2-amino-N-substituted succinimidecompounds is not more than 300 ppm.

We have extensively studied about a cause for discoloration of themaleimide compound in order to solve the above-mentioned problem, tofind that the discoloration of She maleimide compound is not occurred bythe change in the quality of the maleimide compound itself rather thanby the presence of the amines contained in the maleimide compound andused as a raw material for synthesizing the maleimide compound, andfurther that the discoloration speed is considerably enhanced by theexistence of a small amount of chlorine compounds contained in themaleimide compound.

Further, it has also been found that if a volatile organic solventexists in a maleimide compound, the organic solvent in the maleimidecompound evaporates from the surface of the solid maleimide compoundduring its storage and at the same time, the amines and chlorinecompounds contained therein are concentrated on the solid surface,thereby causing the promotion of discoloration of the maleimidecompound.

Furthermore, we have studied about how the extent of discoloring themaleimide compound is effected by each of the components, to find that amaleimide compound excellent in storage stability which is extremelyhard to discolor can be obtained by controlling the content of eachcomponent within a specific range. Thus, the present invention has beencompleted.

BEST MODE FOR CARRYING OUT THE INVENTION

Specifically, the present invention is to provide a maleimide compoundwith improved storage stability wherein the content of primary amines isnot more than 500 ppm. It is preferable that the content of chlorinecompounds reduced as a chlorine atom is not more than 10 ppm or that thecontent of volatile components having a boiling point of not more than200° C. at normal pressure is not more than 2,000 ppm. Further, thepresent invention is to provide a maleimide compound with improvedstorage stability wherein the content of the primary amines is not morethan 500 ppm and the content of maleic anhydride is in the range of 5 to2,000 ppm. Furthermore, the present invention is to provide a maleimidecompound with improved storage stability wherein the content of theprimary amines is not more than 500 ppm and the content of2-amino-N-substituted succinimide compounds is not more than 300 ppm.

The present invention will be explained in more detail as follows. Themaleimide compounds in accordance with the present invention include,for example, N-alkyl maleimides such as N-methyl maleimide, N-ethylmaleimide, N-hexyl maleimide, N-octyl maleimide and N-dodecyl maleimide;N-benzyl maleimides; N-cycloalkyl maleimides such as N-cyclohexylmaleimide; N-phenyl maleimides; N-substituted phenyl maleimides having anitro group, an alkoxyl group, an alkyl group, a carboxyl group, ahydroxyl group or a halogen atom of N-nitrophenyl maleimide,N-methoxyphenyl maleimide, N-methylphenyl maleimide, N-carboxyphenylmaleimide, N-hydroxyphenyl maleimide, N-chlorophenyl maleimide,N-dimethylphenyl maleimide, N-dichlorophenyl maleimide, N-bromophenylmaleimide, N-dibromophenyl maleimide, N-trichlorophenyl maleimide andN-tribromophenyl maleimide and the like substituted, but it should notbe considered that the present invention is limited by these compounds.The present invention is especially effective when aromatic substitutedmaleimide compounds are used.

The essence of the present invention is that the content of primaryamines in the maleimide compound is very low as mentioned above.

In order to improve the storage stability of the maleimide compound justmentioned above, it is necessary to control a content of the primaryamines in the maleimide compound to a level of not more than 500 ppm,preferably in the range of 0.5 to 400 ppm, more preferably in the rangeof 1 to 300 ppm. Further, it is preferable to control a maleic anhydridecontent in the maleimide compound to a level in the range of 5 to 2,000ppm, preferably 10 to 1,000 ppm, more preferably 20 to 400 ppm.Furthermore, it is preferable to control a content of2-amino-N-substituted succinimide compounds in the maleimide compound toa level of not more than 300 ppm, preferably in the range of 0.5 to 200ppm, more preferably in the range of 1 to 120 ppm.

Specifically, the discoloration of the maleimide compound can besuppressed within an allowable range by limiting the content of theprimary amines, or the contents of the primary amines and maleicanhydride or 2-amino-N-substituted succinimide compounds to a level notexceeding the content just mentioned above. If the content of theprimary amines, or the contents of the primary amines and maleicanhydride or 2-amino-N-substituted succinimide compounds exceed theabove-mentioned values, it is not preferable because the discolorationof the maleimide compound increases remarkably.

In addition to the contents of the primary amines and2-amino-N-substituted succinimide compounds, the discoloration of themaleimide compound can be prevented by decreasing a content of chlorinecompounds therein reduced as a chlorine atom to a level of not more than10 ppm, especially not more than 5 ppm. If the chlorine content is notmore than 10 ppm, there is no problem, but if it exceeds the upperlimit, the discoloration rate increases. Further, this chlorine compoundis introduced as a very small amount of the chlorine compound from acatalyst, a solvent, a dehydrator, water or maleic anhydride as a rawmaterial which are used for the production of the maleimide compound.

As these chlorine compounds, alkyl chlorine compounds such as chloroformand carbon tetrachloride, aromatic chlorine compounds such aschlorobenzene and dichlorobenzene, inorganic chlorine compounds such aszinc chloride, sodium chloride, hydrochloric acid and chlorosulfuricacid, and organic phosphate esters having a chlorinated alkyl group anda chlorinated aryl group such as tris(2-chloroethyl) phosphate,tris(dichloropropyl) phosphate, octyl dichloropropyl phosphate andphenyl dichloropropyl phosphate, organic acids such as trichloroaceticacid and sulfinyl chloride can be cited, for example.

Further, if volatile components are contained in a maleimide compound,the primary amines and chlorine compounds contained in the maleimidecompound are locally concentrated with the evaporation of the volatilecomponents. So a part of the maleimide compound in the storage containeris likely to be influenced adversely, such as discolored remarkably. Thevolatile component used herein means a compound having a boiling pointof not more than 200 ° C. at normal pressure. As the typical examplesthereof, benzene, toluene, a petroleum fraction having a boiling pointin the range of 50° to 120 ° C., xylenes, ethyl benzene, isopropylbenzene, cumene, mesitylene, tert-butyl benzene, pseudocumene, trimethylhexane, octane, tetrachloroethane, nonane, chlorobenzene, ethylcyclohexane, a petroleum fraction having a boiling point in the range of120° to 170 ° C., m-dichlorobenzene, sec-butylbenzene,p-dichlorobenzene, decane, p-cymene, o-dichlorobenzene, butyl benzene,decahydronaphthalene, dimethyl formamide, dimethyl acetamide, and thelike can be cited. The content of the volatile components in themaleimide compound is preferably suppressed to an amount of not morethan 2,000 ppm, preferably not more than 1,000 ppm. If it exceeds 2,000ppm, it is more likely to be influenced.

Further, when the maleimide compound is prepared from maleic anhydrideand primary amines, various compounds are usually mixed in the finalmaleimide compound as impurities from raw materials or as byproducts ofthe reaction.

According to our knowledge, the content of the primary amines which areunreacted raw materials varies considerably depending on themanufacturing conditions of the maleimides. For example, if the reactionmolar ratio of maleic anhydride and the primary amines as raw materialsis approximately 1 or the amount of the primary amines is excessive, theprimary amines are contained in the product in a large amount. Further,although a reaction liquid is washed with water in order to purify themaleimide compound produced by the maleimide synthesis reaction, theproduced maleimide compound is sometimes hydrolyzed and produces theprimary amines again which are raw materials. During the production ofthe maleimide compound, impurities by-produced from the primary aminesas raw materials by reaction, purification and the like are sometimesdecomposed with heat and the primary amines as raw materials areliberated again.

Further, the content of the 2-amino-N-substituted succinimide compoundin the final maleimide compound varies considerably. For example, whenthe molar ratio of maleic anhydride and the primary amines isapproximately 1 or the amount of the primary amines is excessive, the2-amino-N-substituted succinimide compound in the product is containedin a large amount. Further, although a reaction liquid is washed withwater in order to purify the maleimide compound produced by themaleimide synthesis reaction, the by-produced impurities are sometimeshydrolyzed to produce an 2-amino-N-substituted succinimide compound. Inorder to suppress the formation of the 2-amino-N-substituted succinimidecompound, the reaction molar ratio of the maleic anhydride and theprimary amines is adapted to be not less than 1 or the2-amino-N-substituted succinimide compound is decomposed by the additionof maleic anhydride at a latter stage of the reaction.

The typical examples of the method for preparing maleimide compound inaccordance with the-present invention will be described as follows:

(1) A method for imidation of maleinamic acids by heat dehydration inthe presence of an acid catalyst using as a diluent a non-polar solventsuch as toluene, xylene, and chlorobenzene or a mixture of a polarsolvent such as dimethyl formamide, dimethyl sulfoxide, N-methylpyrrolidone and sulforane with the non-polar solvent described above.

(2) A method for imidation of maleinamic acids by direct heatdehydration in the presence of an acid catalyst without using.

(3) A method for imidation of maleinamic acids by dehydration using adehydrator such as acetic anhydride.

(4) A method for imidation of maleinamic acids by heat dehydration inthe presence of an acid catalyst and a stabilizer using an organicsolvent as a diluent.

The aforementioned maleinamic acids are obtained by adding and reactingmaleic anhydride with primary amines. In this case, unreacted primaryamines which are contained in maleinamic acids are introduced intoreaction liquids of (1) to (4) described above. With any methoddescribed above, therefore, the amount of the residual primary aminesmixed in the reaction liquid after the dehydration reaction ofmaleinamic acids is preferably as small as possible (preferably in theapproximate range of 1 to 10,000 ppm). So, the reaction conditions foradding and reacting maleic anhydride with primary amines areadvantageously adjusted so as that the unreacted primary amines are notpresent. For this purpose, it is preferable that maleinamic acids areproduced with a molar ratio (M/A) of maleic anhydride (M) and primaryamines (A) of not less than 1, for example. Even when the ratio of thereaction system is out of the aforementioned range, however, the primaryamines, maleic anhydride and 2-amino-N-substituted succinimidecompounds, for example, can be removed in the purification process afterthe aforementioned dehydration process described below, although theburden on the purification process would be large.

Then, the crude maleimide compound obtained by the aforementioneddehydration reaction is subjected to a purification process therebypurified. In this purification process, crystals of maleimide to beseparated out from the reaction liquid are washed With water. Otherwise,the reaction liquid is washed with water in the intact form, andtherefore primary amines, maleic anhydride, fumaric acid, maleinamicacids, and other water-soluble impurities to be contained in thereaction liquid are removed. In this treatment, the crude maleimidecompounds are preferably dissolved in an organic solvent so that thetreatment of washing with water can be more easily carried out. Organicsolvents which can be preferably used herein include benzene, toluene,xylene, mesitylene, cumene, ethylbenzene, cymene, and chlorobenzene, forexample. In this case, a polar solvent such as dimethylformamide,dimethyl sulfoxide, N-methyl pyrrolidone, and sulforane may beadditionally contained therein. It is particularly preferable that theaforementioned organic solvents are used in the manufacturing process ofthe maleimide compound, because the reaction liquid can be washed withwater while it is in the intact form.

In this process of washing the maleimide compound with water, becausethe maleimide compound is easily hydrolyzed by contact with water andprimary amines are reproduced via maleinamic acids, it is necessary topay particular attention to make the time of contact with water as shortas possible. As one example, a treatment of continuously washing themaleimide compound with water which is effective and takes only a shorttime is recommended rather than a treatment which is carried outbatchwisely. Specifically, an organic solvent layer which is a reactionliquid containing a maleimide compound is continuously contacted with anaqueous layer in an air column, a packed column, or a plate column at atemperature in the range of 20° to 90 ° C. for a time in the range of0.1 second to 60 minutes, and the water-soluble components can,therefore, be effectively removed with the primary amines produced bythe hydrolyzation repressed.

Then, the organic solvent is removed by heating from the reaction liquidwhich has been washed with water. In this case, primary amines aredisadvantageously reproduced by heat-decomposition of the impuritiescontained in the crude maleimide compound. The exposure time of themaleimide compound to a high temperature, therefore, is preferably asshort as possible. In this process of removing the solvent, the exposuretime of the maleimide compound to a high temperature is shortened bycarrying out the process of removing the solvent continuously ratherthan batchwisely, similar to that of washing with water. A liquidmembrane downstream type condenser, or a thin membrane evaporator whichfulfills the objects described above can be preferably used, forexample.

As described above, the content of the primary amines is desirably assmall as possible. In the present manufacturing technique andpurification art of maleimide, however, it is economicallydisadvantageous when the content thereof is less than 1 ppm.

The reproduced primary amines are present in the various parts of themanufacturing process of maleimide compound. In order to prevent thecontamination of amines as raw materials in the maleimide compound, eachcondition of the manufacturing process of the maleimide compound must bestudied carefully, to set the conditions which prevent the reproductionof the primary amines as raw materials as far as possible.

In addition to setting the conditions of preventing the reproduction ofthe primary amines as raw materials as far as possible, notableimprovement of the storage stability can be obtained by adjusting thecontent of the maleic anhydride so that the maleic anhydride in theproduct fulfills the specific condition. The amount of the maleicanhydride to be added in the product is necessarily adjusted to a levelin the range of 5 to 2,000 ppm in the maleimide compound of the finalproduct. If the content of the maleic anhydride is less than 5 ppm, thestability effected by the addition of maleic anhydride is unsatisfied.Conversely, it is undesirable to exceed 2,000 ppm, because of theadverse effects on quality such that the heat-resistance of the resinproduct produced with maleimide compound deteriorates, or silverstreakings occur in the molded product. The maleic anhydride to be addedfor the adjustment of the amount of the maleic anhydride in themaleimide compound product can be added in any part of the manufacturingprocess of the maleimide compound. Preferably, it is added after washingwith water, or in the process of obtaining a final product.

In addition to setting the conditions of preventing the reproduction ofthe primary amines as far as possible, notable improvement of thestorage stability can be obtained by adjusting the content of the2-amino-N-substituted succinimide compounds so as that the2-amino-N-substituted succinimide compounds in the product fulfill thespecific condition.

The 2-amino-N-substituted succinimide compound described herein has astructure of the formula as below. ##STR1##

wherein R¹ and R² is an alkyl group having a number of carbon atoms inthe range of 1 to 20; a cycloalkyl group; a phenyl group; a phenyl groupsubstituted with a nitro group, an alkoxy group, a carboxyl group, ahydroxyl group, and a halogen atom, for example.

The amount of the 2-amino-N-substituted succinimide compounds to becontained in the product is necessarily adjusted to be in the range of 1to 300 ppm in the maleimide compound of the final product. If the amountis more than 300 ppm, the discoloration of the maleimide compound duringthe storage is promoted. Although the reason thereof is not clear, it isconsidered that the primary amines are changed into color components bythe interposition of the 2-amino-N-substituted succinimide compound.Further, the 2-amino-N-substituted succinimide compound is notpreferable, because it has adverse effects on the quality of the productsuch that heat-resistance of the resin product produced with maleimidecompound deteriorates, and silver streakings occur in the moldedproduct. The content of the 2-amino-N-substituted succinimide compoundsis preferably as small as possible. If it is less than 0.5 ppm, however,it is not economical because the cost for separating it from a maleimidecompound is unduly high.

Chlorine compounds and volatile components which have the effect ofpromoting the production of color substances from primary amines, orprimary amines and maleic anhydride or 2-amino-N-substituted succinimidecompound must be mixed in the product of the maleimide compound in anamount as small as possible. When a chlorine type compound is needed forthe production with a catalyst, a solvent, a dehydrator, water, andmaleic anhydride as a raw material which are necessary for the synthesisof maleimide compound, for example, chlorine compounds are inevitablymixed therein. Compounds containing no chlorine compounds, therefore,must be used as raw materials in the production of maleimide compound.

As compounds which can be easily mixed during the synthesis of maleimidecompound, and have important effects on the production of colorcomponents, compounds described below can be cited, for example.Specifically, chlorinated alkyl compounds such as chloroform and carbontetrachloride, chlorinated aromatic hydrocarbons such as chlorobenzeneand dichlorobenzene, inorganic chlorine compounds such as zinc chloride,sodium chloride, hydrochloric acid and chlorosulfuric acid, organicphosphate esters having a chlorinated alkyl group and a chlorinated arylgroup such as tris(2-chloroethyl) phosphate, tris(dichloropropyl)phosphate, octyl dichloropropyl phosphate and phenyl dichloropropylphosphate, chlorinated organic acids such as trichloroacetic acid, andchlorinated sulfinyls can be cited, for example.

Volatile components such as reaction solvents must be separated andremoved as much as possible in the purification process of maleimidecompound. In packing a maleimide compound into a product container, itis effective to ventilate a maleimide compound with an inert gas so asto minimize the content of the volatile components.

Maleimide compounds are generally obtained from maleic anhydride andprimary amines by the reactions known to the art. According to theknowledge of the present inventors, maleimide compounds to be producedfrom the two raw materials described above contain primary amines asunreacted raw materials. From these primary amines, a colored substanceis produced during the storage of the aforementioned maleimide compoundsand is a cause of the discoloration of the maleimide compounds. When theamount of the primary amines to be contained in the maleimide compoundis in the range of 1 to 500 ppm, discoloration is not notably shown. Ifthe amount exceeds 500 ppm, the maleimide compound is unduly colored.

By adjusting the amount of the maleic anhydride in the maleimidecompound to a level in the range of 5 to 2,000 ppm, the discoloration ofthe maleimide compound is satisfactorily repressed.

This discoloration is promoted by a small amount of chlorine compoundsintroduced from a catalyst, a solvent, a dehydrator, water or maleicanhydride as a raw material which are used for the production of themaleimide compound, for example. If the chlorine content is not morethan 10 ppm, there are no problems (it is particularly preferable to benot more than 5 ppm). If it exceeds 10 ppm, the speed of thediscoloration is unduly high.

When volatile components are contained in the maleimide compound, sinceprimary amines and chlorine compounds which are contained in themaleimide compound are locally condensed with the evaporation of thesevolatile components, adverse effects such as the notable discolorationof a part of the maleimide compound in a storage container occur. Thevolatile components whose boiling point are not more than 200° C. atnormal pressure are not particularly preferred. When the maleimidecompound contains volatile components in an amount of more than 2,000ppm, the adverse effects occur particularly easily.

The present invention will be described more specifically with workingexamples below.

The N-phenyl maleimides which are to be used in Examples and Controlswere produced by the methods as following.

REFERENTIAL EXAMPLE 1

In an adjustable tank having a volume of 500 liters and provided with astirrer, 200 kg of orthoxylene and 40 kg of orthophosphate were suppliedand 60 kg of diatomaceous earth (trade name; Radiolite #200, produced byShowa Kagaku Kogyo Co., Ltd.) were added thereto, thereby depositingorthophosphate thereon. Then, 19 kg of aniline was added thereto,thereby converting a part of the phosphate to be deposited into aminesalts.

In a reaction tank having a volume of 2 m³ and provided with athermometer, a condenser with a separation tank of water, and a stirrer,110 kg of maleic anhydride and 100 kg of orthoxylene were placed anddissolved. Then, the temperature of the inside of the tank was adjustedto 80° C., and a solution of 100 kg of aniline dissolved in 600 kg oforthoxylene was added quantitatively over 30 minutes, therebysynthesizing an orthoxylene slurry of N-phenyl maleinamic acid.

To this slurry, the total amount of the aforementioned depositedcatalyst and 0.2 kg of copper dibutyl dithiocarbamate were added, andthe resultant mixture was heated. The temperature of the reaction tankwas maintained at 140° C. while stirring. As the water to be producedwith the reaction was removed with orthoxylene, the reaction was carriedout over three hours, to produce N-phenyl maleimide. Thus, a reactionliquid (orthoxylene solution) containing N-phenyl maleimide wasobtained.

The total raw materials to be used contained chlorine in a total amountof 0.4 g. In an apparatus for continuously washing with water providedwith a linemixer (inside diameter of 13.5 mm, trade name; Skeyamixer,and produced by Sakura Seisakusho Co., Ltd.) and a packing column(having 200 mm of inside diameter, 2,000 mm of packing height, and pallring having 1 inch of packing diameter), 300 kg/hr of the reactionliquid thus obtained was continuously contacted with 60 kg/hr of purewater in parallel flow at a temperature of 85° C. in the linemixer as afirst stage, after separating a catalyst therefrom. Then, this mixedsolution was introduced into the lower part of the packing column andseparated. Thereafter, the reaction liquid which had ascended up wassubjected to washing treatment with water as a second stage by beingcontacted with dispersed pure water at a rate of 60 kg/hr at a top ofthe column in an opposite flow direction.

After being washed with water, orthoxylene was removed from the reactionliquid under reduced pressure of 30 mmHg, and further vacuumdistillation was carried out, to obtain 176 kg of yellow N-phenylmaleimide.

REFERENTIAL EXAMPLE 2

In a reaction tank having a volume of 2 m³ and provided with athermometer, a condenser with a separation tank of water, and a stirrer,98 kg of maleic anhydride and 150 liters of toluene, 15 liters ofdimethylsulfoxide and 3 kg of concentrated sulfuric acid were placed anddissolved by heat with stirring. The temperature of the liquid wasmaintained at a temperature of not less than 100° C.

Then, 93 kg of aniline was added to the mixed solution quantitativelyover about 2 hours under reflux of the solvent, and thereafter aged for2 hours at the same temperature. Water which was produced during theaddition and the aging was removed out of the system through theseparation tank of water.

The total raw materials to be used contained chlorine in a total amountof 0.01 g. After the reaction was completed, a yellow and clear reactionliquid was washed batchwisely with 100 kg of pure water at 50° C. in oneportion.

Then, the solvent was removed under reduced pressure of 50 mmHg andfurther vacuum distillation was carried out, to obtain 156 kg of yellowN-phenyl maleimide.

REFERENTIAL EXAMPLE 3

A reaction was carried out under the same conditions with the sameapparatus as used in Referential Example 1, except that the total rawmaterials to be used contained chlorine in a total amount of 0.1 g.Thereafter, the reaction liquid thus obtained and 30 kg of pure wateradded thereto were subjected to washing treatment with water batchwiselyat 85° C. in one portion. Xylene was removed under reduced pressure of50 mmHg from the reaction liquid after being washed with water. Further,vacuum distillation was carried out, to obtain 177 kg of yellow N-phenylmaleimide.

REFERENTIAL EXAMPLE 4

A reaction was carried out under the same conditions with the sameapparatus as used in Referential Example 2, except that the total rawmaterials to be used contained chlorine in a total amount of 0.01 g. Thereaction liquid thus obtained and 100 kg of pure water added theretowere subjected to washing treatment with water batchwisely at 50° C. inone portion. The washed reaction liquid was cooled to a temperature of30° C. to separate crystals and the crystals were filtered off. Then,the solvent was removed under reduced pressure of 50 mmHg from thereaction liquid. Thus, 158 kg of yellow N-phenyl maleimide was obtained.

REFERENTIAL EXAMPLE 5

A reaction was carried out under the same conditions with the sameapparatus as used in Referential Example 1, except that the total rawmaterials to be used contained chlorine in a total amount of 2.5 g. Thereaction liquid thus obtained and 60 kg of pure water added thereto weresubjected to washing treatment with water batchwisely at 50° C. in oneportion. The washed reaction liquid was cooled to a temperature of 30°C. to separate crystals and the crystals were filtered off. Then, xylenewas removed under reduced pressure of 40 mmHg from the reaction liquid.Thus, 179 kg of yellow N-phenyl maleimide was obtained.

EXAMPLES 1 TO 2, AND CONTROLS 1 TO 3

100 kg of N-phenyl maleimide to be prepared by the methods as describedabove and having a shape and impurity composition as shown in Table 1was packed hermetically in a drum made of stainless steel with 59 cm indiameter and 89 cm in height and kept in a room. The storage temperaturewas maintained at a temperature in the range of 10° to 40° C., anddirect sun light was avoided. The storage time was one year and eachN-phenyl maleimide was taken out of the drum every three months and itsappearance was observed. Results as shown in Table 1 were obtained.

                                      TABLE 1    __________________________________________________________________________    Variation of time of N-phenyl    maleimide        Example 1                            Example 2 Control 1                                             Control 2 Control    __________________________________________________________________________                                                       3    Shape            Flaky  Flaky     Flaky  Flaky     Flaky    Composition           Purity                 (wt %)                     not less than                            not less than                                      not less than                                             99.0      99.0                     99.5   99.5      99.5           Aniline                 (ppm)                     10     320       760    980       880           Maleic                 (ppm)                     50     380       50     100       200           anhydride           2-Anilino-N-phenyl                     80     80        80     650       600           succinimide                 (ppm)           Chlorine                 (ppm)                     2      not more than 0.1                                      0.5    not more than 0.1                                                       12    Volatile         Xylene Toluene                                 Dimethyl                                      Xylene Toluene                                                  Dimethyl                                                       Xylene    component                    sulfoxide        sulfoxide           Amount                 (ppm)                     1000   300  1000 4000   400  870  2700    Change of           Before storage                     Bright yellow                            Bright yellow                                      Bright yellow                                             Yellow    Yellow    appearance           After 3 months                     Bright yellow                            Bright yellow                                      Dull yellow                                             Dull yellow                                                       Yellowish brown           After 6 months                     Bright yellow                            Bright yellow                                      Bluish black                                             Yellowish brown                                                       Yellowish brown                                      at upper part                                             totally therein                                                       totally therein                                      of drum          Particularly the                                                       color                                                       of the upper part was                                                       strong           After 9 months                     Bright yellow                            Bright yellow                                      Bluish black                                             Yellowish brown                                                       Brownish at upper                                      at upper part                                             totally therein                                                       part of drum                                      of drum          Particularly the                                                       color                                                       of the upper part was                                                       strong           After 12 months                     Bright yellow                            Bright yellow                                      Bluish black                                             Yellowish brown                                                       Brownish at upper                                      at upper part                                             totally therein                                                       part of drum                                      of drum          Particularly the                                                       color                                                       of the upper part was                                                       strong    Remarks           Purity after 12                     not less than                            not less than                                      not less than                                             99.0      99.0           Purity                 (wt %)                     99.5   99.5      99.5    __________________________________________________________________________

EXAMPLE 3

100 g of N-phenyl maleimide having a shape and impurity composition asshown in Table 2 was placed hermetically in a clear container made ofpolyethylene with 9.5 cm in length and 6.0 cm in width. The N-phenylmaleimide thus packed was stored in a room at a temperature in the rangeof 10° to 40 ° C. with direct sun light avoided. During the storage,each appearance was observed visually.

                  TABLE 2    ______________________________________                    Example 3                             Control 4    ______________________________________    Shape                     Powdery  Powdery                              (Average (Average                              particle particle                              diameter diameter                              200 μm)                                       200 μm)    Composition             Purity   (wt %)  not less than                                       not less than                              99.5     99.5             Aniline  (ppm)   250      640             Chlorine (ppm)   not more than                                       not more than                              0.1      0.1             Xylene   (ppm)   870      3200    Change of             Before storage                          Bright yellow                                     Bright yellow    appearance             After 6 months                          Bright yellow                                     Brownish at                                     upper part                                     Dull yellow                                     totally therein             After 12 months                          Bright yellow                                     Dull yellow                                     totally therein    ______________________________________

CONTROL 5

A reaction was carried out under the same conditions with the sameapparatus as used in Referential Example 2. The reaction liquid thusobtained was not subjected to washing treatment with water. A solventwas removed under reduced pressure of 200 mmHg and further, vacuumdistillation was carried out, to obtain 158 kg of yellow N-phenylmaleimide.

N-phenyl maleimide thus obtained was tested by the same method as usedin Example 3, to obtain results as shown in Table 3.

                  TABLE 3    ______________________________________    Shape              Flaky    ______________________________________    Composition             Purity    (wt %)  98.5             Aniline   (ppm)   1,000             Chlorine  (ppm)   not more than 0.1             Volatile  (ppm)   Toluene      1,000             component         Dimethyl sulfoxide                                            5,000    Change   Before storage                           Yellow    appearance             After 3 months                           Bluish black at upper                           part of drum                           Yellowish brown                           totally therein             After 6 months                           Brownish at upper part                           of drum             After 12 months                           Brownish totally therein    Remarks  Purity after                       (wt %)  98.5             12 months    ______________________________________

EXAMPLE 4 AND CONTROL 6

100 g of N-phenyl maleimide having a shape and impurity composition asshown in Table 4 was packed hermetically in a clear container made ofpolyethylene with 9.5 cm in length and 6.0 cm in width. The N-phenylmaleimide thus packed was stored in a room at a temperature in the rangeof 10° to 40° C. with direct sun light avoided. During the storage, eachappearance was observed visually.

                  TABLE 4    ______________________________________                     Example 4                              Control 6    ______________________________________    Shape                      Powdery  Powdery                               (Average (Average                               particle particle                               diameter diameter                               200 μm)                                        200 μm)    Composition             Purity    (wt %)  not less than                                        not less than                               99.5     99.5             Aniline   (ppm)   200      870             Maleic    (ppm)   420      20             anhydride           2-Aniline-N-phenyl                       30         350           succinimide                   (ppm)           Chlorine                   (ppm)   not more than                                      not more than                           0.1        0.1           Xylene  (ppm)   530        4600    Change of             Before storage                           Bright yellow                                      Bright yellow    appearance             After 6 months                           Bright yellow                                      Brownish at                                      upper half                                      Dull yellow                                      at lower half             After 12 months                           Bright yellow                                      Brownish at                                      upper half                                      Dull yellow                                      at lower half    ______________________________________

INDUSTRIAL APPLICABILITY

As described above, the maleimide compound in accordance with thepresent invention is produced so that the content of primary amines orthe contents of the primary amines and maleic anhydride or2-amino-N-substituted succinimide compounds are controlled to a levelnot exceeding a specific amount and more preferably that the contents ofchlorine compounds and volatile components are controlled to a level notexceeding a specific amount. It, therefore, shows remarkable storagestability. Such maleimide compound with improved storage stability ishighly stable during its transportation and storage, so it does notcause a discoloration in a thermoplastic resin product which is thefinal product.

What is claimed:
 1. A maleimide composition with reduced vulnerabilityto discoloration comprisinga) 99.5 % by weight or more of maleimideselected from the group consisting of N-alkyl maleimides, N-benzylmaleimides, N-cycloalkyl maleimides, N-phenyl maleimides, andN-substituted phenyl maleimides; b) primary amine compounds at a levelnot exceeding 500 ppm; and c) chlorine compounds selected from the groupconsisting of alkyl chlorine compounds, aromatic chlorine compounds,inorganic chlorine compounds, organic phosphate esters having achlorinated alkyl group and a chlorinated aryl group, and organic acids,said chlorine compounds being present at a level not exceeding 10 ppm aschlorine atoms in said maleimide composition.
 2. The maleimidecomposition of claim 1 further comprisingd) volatile compounds, whichhave a boiling point of not more than 200° C., at a level not exceeding2,000 ppm; e) maleic anhydride at a level not exceeding 5 to 2000 ppm;and f) 2-amino-N-substituted succinimide compounds at a level notexceeding 300 ppm.
 3. The maleimide composition of claim 1 wherein saidmaleimide is in a solid state.
 4. The maleimide composition of claim 1wherein said maleimide is an N-substituted phenyl maleimide.
 5. Aprocess of suppressing discoloration of maleimide compositionscomprising 99.5% by weight or more of maleimide, said processcomprisinga) limiting the content of primary amines in said compositionto a level not exceeding 500 ppm; b) limiting the content of chlorinecompounds in said composition to a level not exceeding 10 ppm; c)limiting the content of volatile compounds in said composition to alevel not exceeding 2,000 ppm; d) limiting the content of maleicanhydride in said composition to a level not exceeding 500 ppm; and e)limiting the content of 2-amino-N-substituted succinimide compounds to alevel of not more than 300 ppm.
 6. A process of suppressing the level of2-amino-N-substituted succinimide compound in the maleimide compositionof claim 5 comprising reacting maleic anhydride and primary amine in amolar ratio of not less than 1 in order to suppress formation of2-amino-N-substituted succinimide compound.
 7. The process of claim 5wherein limiting the content of volatile compounds is performed byventilating solid maleimide compound with an inert gas.